[feaLib] Support compact syntax for chaining to ligature substitutions

https://github.com/behdad/fonttools/issues/445

Lib/fontTools/feaLib/ast.py

@@ -250,19 +250,17 @@ class IgnoreSubstitutionRule(Statement):
 
 
 class LigatureSubstStatement(Statement):
-    def __init__(self, location, glyphs, replacement):
+    def __init__(self, location, prefix, glyphs, suffix, replacement):
         Statement.__init__(self, location)
-        self.glyphs, self.replacement = (glyphs, replacement)
+        self.prefix, self.glyphs, self.suffix = (prefix, glyphs, suffix)
+        self.replacement = replacement
 
     def build(self, builder):
-        # OpenType feature file syntax, section 5.d, "Ligature substitution":
-        # "Since the OpenType specification does not allow ligature
-        # substitutions to be specified on target sequences that contain
-        # glyph classes, the implementation software will enumerate
-        # all specific glyph sequences if glyph classes are detected"
-        g = [g.glyphSet() for g in self.glyphs]
-        for glyphs in sorted(itertools.product(*g)):
-            builder.add_ligature_subst(self.location, glyphs, self.replacement)
+        prefix = [p.glyphSet() for p in self.prefix]
+        glyphs = [g.glyphSet() for g in self.glyphs]
+        suffix = [s.glyphSet() for s in self.suffix]
+        builder.add_ligature_subst(
+            self.location, prefix, glyphs, suffix, self.replacement)
 
 
 class LookupFlagStatement(Statement):

Lib/fontTools/feaLib/builder.py

@@ -4,6 +4,7 @@ from fontTools.feaLib.error import FeatureLibError
 from fontTools.feaLib.parser import Parser
 from fontTools.ttLib import getTableClass
 from fontTools.ttLib.tables import otBase, otTables
+import itertools
 import warnings
 
 
@@ -410,9 +411,22 @@ class Builder(object):
                 location)
         lookup.alternates[glyph] = from_class
 
-    def add_ligature_subst(self, location, glyphs, replacement):
-        lookup = self.get_lookup_(location, LigatureSubstBuilder)
-        lookup.ligatures[glyphs] = replacement
+    def add_ligature_subst(self, location,
+                           prefix, glyphs, suffix, replacement):
+        if prefix or suffix:
+            lookup = self.get_chained_lookup_(location, LigatureSubstBuilder)
+            chain = self.get_lookup_(location, ChainContextSubstBuilder)
+            chain.substitutions.append((prefix, glyphs, suffix, [lookup]))
+        else:
+            lookup = self.get_lookup_(location, LigatureSubstBuilder)
+
+        # OpenType feature file syntax, section 5.d, "Ligature substitution":
+        # "Since the OpenType specification does not allow ligature
+        # substitutions to be specified on target sequences that contain
+        # glyph classes, the implementation software will enumerate
+        # all specific glyph sequences if glyph classes are detected"
+        for g in sorted(itertools.product(*glyphs)):
+            lookup.ligatures[g] = replacement
 
     def add_multiple_subst(self, location,
                            prefix, glyph, suffix, replacements):

Lib/fontTools/feaLib/builder_test.py

@@ -16,7 +16,7 @@ import unittest
 
 def makeTTFont():
     glyphs = (
-        ".notdef space slash fraction semicolon period comma "
+        ".notdef space slash fraction semicolon period comma ampersand "
         "zero one two three four five six seven eight nine "
         "zero.oldstyle one.oldstyle two.oldstyle three.oldstyle "
         "four.oldstyle five.oldstyle six.oldstyle seven.oldstyle "
@@ -26,7 +26,7 @@ def makeTTFont():
         "A.sc B.sc C.sc D.sc E.sc F.sc G.sc H.sc I.sc J.sc K.sc L.sc M.sc "
         "N.sc O.sc P.sc Q.sc R.sc S.sc T.sc U.sc V.sc W.sc X.sc Y.sc Z.sc "
         "A.alt1 A.alt2 A.alt3 B.alt1 B.alt2 B.alt3 C.alt1 C.alt2 C.alt3 "
-        "d.alt n.end s.end "
+        "d.alt e.begin n.end s.end "
         "f_l c_h c_k c_s c_t f_f f_f_i f_f_l f_i o_f_f_i s_t "
         "ydieresis yacute "
         "grave acute dieresis macron circumflex cedilla umlaut ogonek caron "
@@ -165,7 +165,7 @@ class BuilderTest(unittest.TestCase):
             self.expect_ttx(font, self.getpath("GSUB_%s.ttx" % name))
 
     def test_spec(self):
-        for name in "4h1 5d1 5d2 5fi1 5fi2 5h1 6d2 6e 6f 6h_ii".split():
+        for name in "4h1 5d1 5d2 5fi1 5fi2 5fi4 5h1 6d2 6e 6f 6h_ii".split():
             font = makeTTFont()
             addOpenTypeFeatures(self.getpath("spec%s.fea" % name), font)
             self.expect_ttx(font, self.getpath("spec%s.ttx" % name))

Lib/fontTools/feaLib/parser.py

@@ -510,12 +510,13 @@ class Parser(object):
 
         # GSUB lookup type 4: Ligature substitution.
         # Format: "substitute f f i by f_f_i;"
-        if (not reverse and len(old_prefix) == 0 and len(old_suffix) == 0 and
+        if (not reverse and
                 len(old) > 1 and len(new) == 1 and
                 len(new[0].glyphSet()) == 1 and
                 num_lookups == 0):
             return ast.LigatureSubstStatement(
-                location, old, list(new[0].glyphSet())[0])
+                location, old_prefix, old, old_suffix,
+                list(new[0].glyphSet())[0])
 
         # GSUB lookup type 8: Reverse chaining substitution.
         if reverse:

Lib/fontTools/feaLib/parser_test.py

@@ -807,7 +807,7 @@ class ParserTest(unittest.TestCase):
         self.assertEqual(sub.glyph, "f_f_i")
         self.assertEqual(sub.replacement, ("f", "f", "i"))
 
-    def test_substitute_multiple_chained(self):  # GSUB LookupType 2
+    def test_substitute_multiple_chained(self):  # chain to GSUB LookupType 2
         doc = self.parse("lookup L {sub [A-C] f_f_i' [X-Z] by f f i;} L;")
         sub = doc.statements[0].statements[0]
         self.assertIsInstance(sub, ast.MultipleSubstStatement)
@@ -836,9 +836,20 @@ class ParserTest(unittest.TestCase):
     def test_substitute_ligature(self):  # GSUB LookupType 4
         doc = self.parse("feature liga {substitute f f i by f_f_i;} liga;")
         sub = doc.statements[0].statements[0]
-        self.assertEqual(type(sub), ast.LigatureSubstStatement)
+        self.assertIsInstance(sub, ast.LigatureSubstStatement)
         self.assertEqual(glyphstr(sub.glyphs), "f f i")
         self.assertEqual(sub.replacement, "f_f_i")
+        self.assertEqual(glyphstr(sub.prefix), "")
+        self.assertEqual(glyphstr(sub.suffix), "")
+
+    def test_substitute_ligature_chained(self):  # chain to GSUB LookupType 4
+        doc = self.parse("feature F {substitute A B f' i' Z by f_i;} F;")
+        sub = doc.statements[0].statements[0]
+        self.assertIsInstance(sub, ast.LigatureSubstStatement)
+        self.assertEqual(glyphstr(sub.glyphs), "f i")
+        self.assertEqual(sub.replacement, "f_i")
+        self.assertEqual(glyphstr(sub.prefix), "A B")
+        self.assertEqual(glyphstr(sub.suffix), "Z")
 
     def test_substitute_lookups(self):  # GSUB LookupType 6
         doc = Parser(self.getpath("spec5fi1.fea")).parse()

Lib/fontTools/feaLib/testdata/GSUB_6.fea

@@ -7,7 +7,12 @@ lookup ChainedMultipleSubst {
     substitute [A-C a-c] [D d] E c_t' V [W w] [X-Z x-z] by c t;
 } ChainedMultipleSubst;
 
+lookup ChainedLigatureSubst {
+    substitute A [C c]' [T t]' Z by c_t;
+} ChainedLigatureSubst;
+
 feature test {
     lookup ChainedSingleSubst;
     lookup ChainedMultipleSubst;
+    lookup ChainedLigatureSubst;
 } test;

Lib/fontTools/feaLib/testdata/GSUB_6.ttx

@@ -22,14 +22,15 @@
       <FeatureRecord index="0">
         <FeatureTag value="test"/>
         <Feature>
-          <!-- LookupCount=2 -->
+          <!-- LookupCount=3 -->
           <LookupListIndex index="0" value="1"/>
           <LookupListIndex index="1" value="4"/>
+          <LookupListIndex index="2" value="6"/>
         </Feature>
       </FeatureRecord>
     </FeatureList>
     <LookupList>
-      <!-- LookupCount=5 -->
+      <!-- LookupCount=7 -->
       <Lookup index="0">
         <!-- LookupType=1 -->
         <LookupFlag value="0"/>
@@ -151,6 +152,50 @@
           </SubstLookupRecord>
         </ChainContextSubst>
       </Lookup>
+      <Lookup index="5">
+        <!-- LookupType=4 -->
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <LigatureSubst index="0" Format="1">
+          <LigatureSet glyph="C">
+            <Ligature components="T" glyph="c_t"/>
+            <Ligature components="t" glyph="c_t"/>
+          </LigatureSet>
+          <LigatureSet glyph="c">
+            <Ligature components="T" glyph="c_t"/>
+            <Ligature components="t" glyph="c_t"/>
+          </LigatureSet>
+        </LigatureSubst>
+      </Lookup>
+      <Lookup index="6">
+        <!-- LookupType=6 -->
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <ChainContextSubst index="0" Format="3">
+          <!-- BacktrackGlyphCount=1 -->
+          <BacktrackCoverage index="0">
+            <Glyph value="A"/>
+          </BacktrackCoverage>
+          <!-- InputGlyphCount=2 -->
+          <InputCoverage index="0">
+            <Glyph value="C"/>
+            <Glyph value="c"/>
+          </InputCoverage>
+          <InputCoverage index="1">
+            <Glyph value="T"/>
+            <Glyph value="t"/>
+          </InputCoverage>
+          <!-- LookAheadGlyphCount=1 -->
+          <LookAheadCoverage index="0">
+            <Glyph value="Z"/>
+          </LookAheadCoverage>
+          <!-- SubstCount=1 -->
+          <SubstLookupRecord index="0">
+            <SequenceIndex value="0"/>
+            <LookupListIndex value="5"/>
+          </SubstLookupRecord>
+        </ChainContextSubst>
+      </Lookup>
     </LookupList>
   </GSUB>
 

Lib/fontTools/feaLib/testdata/spec5fi4.fea

@@ -0,0 +1,9 @@
+# OpenType Feature File specification, section 5.f.i, example 4
+# "Specifying a Chain Sub rule and marking sub-runs"
+# http://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html
+
+languagesystem latn dflt;
+
+feature test {
+    substitute [e e.begin]' t' c by ampersand;
+} test;

Lib/fontTools/feaLib/testdata/spec5fi4.ttx

@@ -0,0 +1,73 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ttFont>
+
+  <GSUB>
+    <Version value="1.0"/>
+    <ScriptList>
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="latn"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
+    </ScriptList>
+    <FeatureList>
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="test"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="1"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=2 -->
+      <Lookup index="0">
+        <!-- LookupType=4 -->
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <LigatureSubst index="0" Format="1">
+          <LigatureSet glyph="e">
+            <Ligature components="t" glyph="ampersand"/>
+          </LigatureSet>
+          <LigatureSet glyph="e.begin">
+            <Ligature components="t" glyph="ampersand"/>
+          </LigatureSet>
+        </LigatureSubst>
+      </Lookup>
+      <Lookup index="1">
+        <!-- LookupType=6 -->
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <ChainContextSubst index="0" Format="3">
+          <!-- BacktrackGlyphCount=0 -->
+          <!-- InputGlyphCount=2 -->
+          <InputCoverage index="0">
+            <Glyph value="e"/>
+            <Glyph value="e.begin"/>
+          </InputCoverage>
+          <InputCoverage index="1">
+            <Glyph value="t"/>
+          </InputCoverage>
+          <!-- LookAheadGlyphCount=1 -->
+          <LookAheadCoverage index="0">
+            <Glyph value="c"/>
+          </LookAheadCoverage>
+          <!-- SubstCount=1 -->
+          <SubstLookupRecord index="0">
+            <SequenceIndex value="0"/>
+            <LookupListIndex value="0"/>
+          </SubstLookupRecord>
+        </ChainContextSubst>
+      </Lookup>
+    </LookupList>
+  </GSUB>
+
+</ttFont>